The RAT allows the blue system to be pressurised which in turn runs the CSMG (constant speed motor generator) its a hydraulic motor with a generator on its output.
It is controlled by its own GCU which maintains the speed of the motor by varying a swash plate within it. Which in turn maintains the output of the generator at 115Vac, 400Hz.
Depending on what it delievering pressure to the blue system also determines the output available in kVA of the CSMG. When its the RAT the output is its lowest.
Jimpy